In the inkjet recording method, ink droplets are ejected to adhere to a recording medium such as paper. An ink used in the inkjet recording method usually contains various colorants, such as dyes and pigments, dissolved or dispersed in water or in water and high-boiling point organic solvents, and the ink further contains humectants including high boiling point organic solvents for sustaining moisture retaining properties. As such the humectants have a low volatility (evaporation properties) and water retention properties, the humectants including high-boiling point organic solvents are useful for preventing nozzles from drying out. However, strike-through of the ink may frequently occur when highly absorbent paper such as regular paper is used in printing because the humectants do not evaporate quickly.
On the other hand, in applications using less absorbent paper such as coated offset printing paper (termed “the coated paper” on some occasions hereafter), the prior art ink containing a large amount of humectants often causes beading and bleeding in solid color areas where a large amount of ink is applied because the ink applied on the surface of the coated paper slowly infiltrates therein. Furthermore, because the humectants do not evaporate quickly, there are some impractical issues such as longer drying time, smeared offset or conveyers by spurs, smeared paper or paper stuck together in the discharged paper stack.
Then, there have been demands for an inkjet recording method that realizes reduced beading, improved drying speed, high image densities, and high quality even on regular paper in printing on coated printing paper and that does not cause clogged nozzles after a prolonged unused period.
For example, Patent literature 1 proposes an inkjet ink containing an extremely small amount of humectants. This proposal realizes high image densities on regular paper without strike-through. However, because the ink contains no resins, the image is not fixed well particularly when pigmented ink is used. There is no disclosure or suggestion as to improvement in drying speed upon printing on coated offset printing paper.
Patent literature 2 proposes a quick-drying dye ink. Embodiments of this proposal contain humectants in amounts six times higher than colorants and no resins. In the embodiments, high quality paper is used and poorly absorbent coated printing paper is excluded.
Patent literature 3 proposes a quick-drying ink for electro-exothermic inkjet systems. This proposal also contains no resins and a large amount of electrolytes, leading to poorly fixed images. Here, the ejection system is used, which is substantially different from the piezo inkjet system.
Patent literature 4 and Patent literature 5 propose combinations of microcapsulated pigments and printing paper. These proposals use conventional ink compositions and require heating by microwaves for improved drying properties.
Patent literature 6 proposes an ink containing wax particles and resin particles. In this proposal, the wax particles can contribute to moisturizing and are used in place of humectants. However, the wax particles having moisturizing properties usually are less useful for fixing and drying.
Patent literature 7 proposes an ink containing resin emulsions, high-molecular weight dispersants, and pigments. This proposed ink contains a small amount of humectants. However, because of the low total solid content, problems such as insufficient image densities and color development occur.
Patent literature 8 has proposed an ink suitable for non-porous recording media such as films, the ink containing an ink resin emulsion containing an aqueous liquid vehicle, acid-functional polymer colloid particles dispersed in the aqueous liquid vehicle, and polymer-coupled pigment colorants, high-molecular weight dispersants, and pigments. Embodiments of this proposal contain high-boiling point humectants (water-soluble organic solvents) in amounts approximately 3.5 times higher than the solid content. It dries slow upon standing in general environments after recording. Therefore, heating by a heat gun is necessary.
As described above, the prior art inkjet recording ink is not considered to be used with less absorbent recording media such as coated printing paper including coated offset printing paper. No techniques are offered to reduce the amount of the liquid constituents (principally humectants) having boiling points higher than water and being liquid in the ink at 25° C. as a result of focusing on the ratio to the solid constituents for improved ink infiltration and to prevent problematic clogged nozzles after a prolonged unused period.
[Patent literature 1]
    Japanese Patent Application Laid-Open (JP-A) No. 2004-115551[Patent literature 2]    Japanese Patent Application Publication (JP-B) No. 60-34992[Patent literature 3]    Japanese Patent Application Laid-Open (JP-A) No. 08-109343[Patent literature 4]    Japanese Patent Application Laid-Open (JP-A) No. 2002-67473[Patent literature 5]    Japanese Patent Application Laid-Open (JP-A) No. 2002-69346[Patent literature 6]    Japanese Patent Application Laid-Open (JP-A) No. 2002-301857[Patent literature 7]    Japanese Patent Application Laid-Open (JP-A) No. 06-171072[Patent literature 8]    Japanese Patent Application Laid-Open (JP-A) No. 2005-220352